This invention relates to pneumatic tires and, more particularly, to a novel and highly-effective pneumatic tire, with or without a separate inner tube, which has a carcass reinforcement formed of radial cords or cables.
In general, a radial carcass reinforcement is used in combination with a tread reinforcement formed of at least two plies of cords or cables which are parallel in each ply and crossed from one ply to the next. The primary function of the tread reinforcement is to stiffen the tread parallel to the area of contact with the ground. For this reason the cords or cables of the tread plies form small angles with the longitudinal direction of the tire. Such an arrangement of the tread plies tends to impose a circumference close to that of the tread reinforcement in the longitudinal direction of the tire. Furthermore, in the side walls, the carcass reinforcement assumes a more convex profile than it would in the absence of a tread reinforcement. This can be seen by comparing, at the same inflation pressure, the equilibrium profile of a radial carcass belted by a tread reinforcement and the equilibrium profile assumed by the same carcass without a tread reinforcement. The side walls of the latter are less convex, but the tread shows at the same time increased curvature in radial planes of the tire and therefore an increased circumference in the longitudinal direction of the tire. Such an equilibrium profile of a carcass without a tread reinforcement, referred to as the natural or free equilibrium profile, can be represented by the equation EQU cos .phi. = (R.sup.2 - R.sub.e.sup.2)/(R.sub.s.sup.2 - R.sub.e.sup.2) (1)
In this equation .phi. is the angle formed by (1) the tangent to the equilibrium profile and (2) the axis of rotation of the tire, the tangent point being at a radius R from said axis; R.sub.e is the radius of the equilibrium profile corresponding to the maximum axial width reached in the side walls by this profile; and R.sub.s is the radius of the point of the tread farthest from the axis of rotation located on the axis of symmetry, that is to say in the equatorial plane of the carcass.
It is only recently, in U.S. Pat. No. 3,757,844 to Henri Verdier, for "High-Speed Radial Tire", issued Sept. 11, 1973, that there has been recognized the importance of having a tread reinforcement which respects the natural equilibrium profile assumed by the carcass in the absence of a tread. The object of the arrangement described in that patent is to remedy the unfavorable state of stresses between the carcass reinforcement and the tread reinforcement. For this reason, this patent provides that the carcass assumes its own equilibrium profile only in the region of the side walls, or, otherwise stated, in a region in which the carcass is removed from the stiffening action both of the beads and of the tread reinforcement. In order to arrive at this result, the tread reinforcement contains especially stiffened edges. With regard to the reinforcements of the beads, they impose upon the carcass reinforcement, starting from the radius where the latter is subject to their influence, a substantially linear profile or even a profile curved in the direction opposite the curvature in the side walls, that is to say a point of inflection followed by a curving in the same direction as the edges of the rim.
Such an arrangement was an important advance in the art of tire manufacture but is insufficient when the diameter of such tires is greatly decreased without decreasing the standardized diameter and width of the rims on which they are to be mounted. Therefore, the height H on the rim, that is to say the difference between the outer diameter of the carcass reinforcement and the diameter thereof at the level of the bead, is relatively small as compared with the greatest axial width B of the carcass reinforcement, so that the aspect ratio H/B is frequently less than 0.6. Under these circumstances, and particularly when using a tread reinforcement which has a width either close to that of the rim or greater than it, the length of the cords or cables of the carcass reinforcement connecting the beads to the tread reinforcement and passing through the front and rear ends of the tire footprint or area of contact of the tire with the ground becomes insufficient. Therefore, the tread reinforcement can no longer assume its customary longitudinal length in the area of contact, and the latter is thereby shortened. This results in stress distributions and heating at the areas of the connection between the carcass reinforcement and the reinforced edges of the tread reinforcement on the one hand and the beads on the other hand, which are harmful to the life of these junction areas, particularly when the tires are traveling under higher loads and/or with inflation pressures which are lower than the rated values. Furthermore, the reduction in size imposed upon the surface of the contact area results in a certain reduction in the resistance to wear, the comfort, and the road-holding ability of the tires.
Contrary to what might be believed, the remedy for this state of affairs does not consist in deliberately increasing the length of the cords or cables of the carcass reinforcement between the upper end of the beads and the tread reinforcement. This would mean going back to the state of the art prior to the patent cited above. As a matter of fact, by doing this one would aggravate the distribution of the stresses at the level of the junctions between the carcass reinforcement, the tread reinforcement and the beads.